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Hormones and Glands
Pituitary gland
The animal body has two levels of
coordination: nervous coordination
and chemical coordination. Chemical
coordination is centered in a system
of glands known as endocrine
glands. These glands are situated
throughout the animal body and
include such organs as the pancreas,
thyroid gland, and adrenal gland.
The glands secrete hormones, a
series of chemical substances
composed of protein or sterol lipids.
The pituitary gland is located at the
base of the human brain. The gland
consists of two parts: the anterior
lobe (adenohypophysis) and the
posterior lobe (neurohypophysis).
Hormones bring about changes that
help coordinate body systems in a
general way. For example, the
pancreas secretes insulin, which
facilitates the passage of glucose
into all body cells for use in energy
metabolism. Another example is
thyroxine, a thyroid gland secretion
that regulates overall body
metabolism. In contrast to chemical
coordination, the nervous system
coordinates functions in the animal
body on a more localized level as it
delivers nerve impulses to contract
body muscles or regulate gland
activities.
The endocrine glands secrete their
hormones into the bloodstream,
where the blood carries the
hormones to the target organs.
Because the endocrine glands have
no ducts, they are often called
ductless glands. Other glands of the
body (such as the enzyme-secreting
salivary glands) deliver their
enzymes via ducts and are referred
to as exocrine glands.
Human Endocrine System
The human endocrine system
modulates several processes of the
body by the function of hormones.
The endocrine system secretes
hormones that control how bodily
functions work. Thus, the human
endocrine system watches over and
coordinates all the systems of the
body by the use of hormones.
The anterior lobe secretes at least
seven hormones. One hormone, the
human growth hormone (HGH),
promotes body growth by
accelerating protein synthesis. This
hormone is also known as
somatotropin. A deficiency of the
hormone results in dwarfism; an
oversecretion results in gigantism.
Another hormone of the anterior
pituitary is prolactin, also called
lactogenic hormone (LH). This
hormone promotes breast
development and milk secretion in
females. A third hormone is thyroidstimulating hormone (TSH). The
function of TSH is to control
secretions of hormones from the
thyroid gland. A fourth hormone is
adrenocorticotropic hormone
(ACTH). This hormone controls the
secretion of hormones from the
adrenal glands.
There are three more hormones
produced in the anterior lobe of the
pituitary gland. The first is folliclestimulating hormone (FSH). In
females, FSH stimulates the
development of a follicle, which
contains the egg cell; in males, the
hormone stimulates sperm
production. The next hormone is
luteinizing hormone (LH). In females,
LH completes the maturation of the
follicle and stimulates the formation
of the corpus luteum, which
temporarily secretes female
hormones. In males, LH is interstitial
cell-stimulating hormone (ICSH),
which stimulates the production of
male hormones in the testes. The
final hormone is melanocytestimulating hormone (MSH), which
stimulates production of the
pigment melanin.
The posterior pituitary gland stores
and then releases two hormones
that are produced in the
hypothalamus of the brain. The first
hormone is antidiuretic hormone
(ADH). This hormone stimulates
water reabsorption in the kidneys. It
is also called vasopressin. The
second hormone is oxytocin, which
stimulates contractions in the
muscles of the uterus during birth.
Thyroid gland
The thyroid gland lies against the
pharynx at the base of the neck. It
consists of two lateral lobes
connected by an isthmus. The gland
produces thyroxine, a hormone that
regulates the rate of metabolism in
the body. It also produces a second
hormone, calcitonin, which regulates
the level of calcium in the blood.
Thyroxine production depends on
the availability of iodine. A
deficiency of iodine causes thyroid
gland enlargement, a condition
called goiter. An undersecretion of
thyroxine results in a condition
known as cretinism (dwarfism with
abnormal body proportions and
possible mental retardation). In
adults, an undersecretion results in
myxedema (physical and mental
sluggishness). Thyroxine
oversecretion results in a high
metabolic rate and Graves' disease.
Parathyroid glands
The parathyroid glands are located
on the posterior surfaces of the
thyroid gland. They are tiny masses
of glandular tissue that produce
parathyroid hormone, also called
parathormone. Parathyroid
hormone regulates calcium
metabolism in the body by
increasing calcium reabsorption in
the kidneys, and by increasing the
uptake of calcium from the digestive
system.
Adrenal glands
The adrenal glands are two
pyramid-shaped glands lying atop
the kidneys. The adrenal glands
consist of an outer portion, the
cortex, and an inner portion, the
medulla.
The adrenal cortex secretes a family
of steroids called corticosteroids.
The two main types of steroid
hormones are mineralocorticoids
and glucocorticoids.
Mineralocorticoids, such as
aldosterone, control mineral
metabolism in the body. They
accelerate mineral reabsorption in
the kidney. Mineralocorticoid
secretion is regulated by ACTH from
the pituitary gland. Glucocorticoids,
such as cortisol and cortisone,
control glucose metabolism and
protein synthesis in the body.
Glucocorticoids are also antiinflammatory agents.
The adrenal medulla produces two
hormones: epinephrine (adrenaline)
and norepinephrine (noradrenaline).
Epinephrine increases heart rate,
blood pressure, and the blood
supply to skeletal muscle.
Epinephrine functions in stressful
situations to promote the fight–
flight response. Norepinephrine
intensifies the effects of
epinephrine. Both hormones
prolong and intensify the effects of
the sympathetic nervous system.
Pancreas
The pancreas is located just behind
the stomach. Its endocrine portion
consists of cell clusters called the
islets of Langerhans.
The pancreas produces two
hormones: insulin and glucagon.
Insulin is a protein that promotes
the passage of glucose molecules
into the body cells and regulates
glucose metabolism. In the absence
of insulin, glucose is removed from
the blood and excreted in the
kidney, a condition called diabetes
mellitus. Diabetes mellitus is
characterized by glucose in the
urine, heavy urination, excessive
thirst, and a generally sluggish body
metabolism.
The second pancreatic hormone,
glucagon, stimulates the breakdown
of glycogen to glucose in the liver. It
also releases fat from the adipose
tissue so the fat can be used for the
production of carbohydrates.
Other endocrine glands
Among the other endocrine glands
are the ovaries and testes. The
ovaries secrete estrogens, which
encourage the development of
secondary female characteristics.
The testes secrete androgens, which
promote secondary male
characteristics. Testosterone is an
important androgen.
The pineal gland is a tiny gland in
the midbrain. Its functions are
largely unknown, but it seems to
regulate mating behaviors and day–
night cycles. The thymus gland is
located in the neck tissues. It
secretes thymosins, which influence
the development of the Tlymphocytes of the immune system.
Prostaglandins are hormones
secreted by various tissue cells.
These hormones produce their
effects on smooth muscles, on
various glands, and in reproductive
physiology. Erythropoietin is a
hormone produced by the kidney
cells. Erythropoietin functions in the
production of red blood cells.
Gastrin and secretin are hormones
produced by digestive glands to
influence digestive processes.